Import Statement

Availability

See Also

Returns a Boolean value that indicates whether the receiver and a given object are equal. (required)

Declaration

Swift

funcisEqual(_anObject: AnyObject?) -> Bool

Objective-C

- (BOOL)isEqual:(id)anObject

Parameters

anObject

The object to be compared to the receiver. May be nil, in which case this method returns NOfalse.

Return Value

YEStrue if the receiver and anObject are equal, otherwise NOfalse.

Discussion

This method defines what it means for instances to be equal. For example, a container object might define two containers as equal if their corresponding objects all respond YEStrue to an isEqual: request. See the NSData, NSDictionary, NSArray, and NSString class specifications for examples of the use of this method.

If two objects are equal, they must have the same hash value. This last point is particularly important if you define isEqual: in a subclass and intend to put instances of that subclass into a collection. Make sure you also define hash in your subclass.

See Also

Declaration

Return Value

An integer that can be used as a table address in a hash table structure.

Discussion

If two objects are equal (as determined by the isEqual: method), they must have the same hash value. This last point is particularly important if you define hash in a subclass and intend to put instances of that subclass into a collection.

If a mutable object is added to a collection that uses hash values to determine the object’s position in the collection, the value returned by the hash method of the object must not change while the object is in the collection. Therefore, either the hash method must not rely on any of the object’s internal state information or you must make sure the object’s internal state information does not change while the object is in the collection. Thus, for example, a mutable dictionary can be put in a hash table but you must not change it while it is in there. (Note that it can be difficult to know whether or not a given object is in a collection.)

Availability

See Also

Returns a Boolean value that indicates whether the receiver is an instance of given class or an instance of any class that inherits from that class. (required)

Declaration

Swift

funcisKindOfClass(_aClass: AnyClass) -> Bool

Objective-C

- (BOOL)isKindOfClass:(Class)aClass

Parameters

aClass

A class object representing the Objective-C class to be tested.

Return Value

YEStrue if the receiver is an instance of aClass or an instance of any class that inherits from aClass, otherwise NOfalse.

Discussion

For example, in this code, isKindOfClass: would return YEStrue because, in Foundation, the NSArchiver class inherits from NSCoder:

NSMutableData*myData=[NSMutableDatadataWithCapacity:30];

idanArchiver=[[NSArchiveralloc]initForWritingWithMutableData:myData];

if([anArchiverisKindOfClass:[NSCoderclass]])

...

Be careful when using this method on objects represented by a class cluster. Because of the nature of class clusters, the object you get back may not always be the type you expected. If you call a method that returns a class cluster, the exact type returned by the method is the best indicator of what you can do with that object. For example, if a method returns a pointer to an NSArray object, you should not use this method to see if the array is mutable, as shown in the following code:

// DO NOT DO THIS!

if([myArrayisKindOfClass:[NSMutableArrayclass]])

{

// Modify the object

}

If you use such constructs in your code, you might think it is alright to modify an object that in reality should not be modified. Doing so might then create problems for other code that expected the object to remain unchanged.

If the receiver is a class object, this method returns YEStrue if aClass is a Class object of the same type, NOfalse otherwise.

Import Statement

Availability

See Also

Returns a Boolean value that indicates whether the receiver implements or inherits a method that can respond to a specified message. (required)

Declaration

Swift

funcrespondsToSelector(_aSelector: Selector) -> Bool

Objective-C

- (BOOL)respondsToSelector:(SEL)aSelector

Parameters

aSelector

A selector that identifies a message.

Return Value

YEStrue if the receiver implements or inherits a method that can respond to aSelector, otherwise NOfalse.

Discussion

The application is responsible for determining whether a NOfalse response should be considered an error.

You cannot test whether an object inherits a method from its superclass by sending respondsToSelector: to the object using the super keyword. This method will still be testing the object as a whole, not just the superclass’s implementation. Therefore, sending respondsToSelector: to super is equivalent to sending it to self. Instead, you must invoke the NSObject class method instancesRespondToSelector: directly on the object’s superclass, as illustrated in the following code fragment.

if([MySuperclassinstancesRespondToSelector:@selector(aMethod)]){

// invoke the inherited method

[superaMethod];

}

You cannot simply use [[self superclass] instancesRespondToSelector:@selector(aMethod)] since this may cause the method to fail if it is invoked by a subclass.

Note that if the receiver is able to forward aSelector messages to another object, it will be able to respond to the message, albeit indirectly, even though this method returns NOfalse.

Availability

See Also

Returns a Boolean value that indicates whether the receiver conforms to a given protocol. (required)

Declaration

Swift

funcconformsToProtocol(_aProtocol: Protocol) -> Bool

Objective-C

- (BOOL)conformsToProtocol:(Protocol *)aProtocol

Parameters

aProtocol

A protocol object that represents a particular protocol.

Return Value

YEStrue if the receiver conforms to aProtocol, otherwise NOfalse.

Discussion

This method works identically to the conformsToProtocol: class method declared in NSObject. It’s provided as a convenience so that you don’t need to get the class object to find out whether an instance can respond to a given set of messages.

Declaration

Return Value

A string that describes the contents of the receiver for presentation in the debugger.

Discussion

The debugger’s print-object command invokes this method to produce a textual description of an object.

NSObject implements this method by calling through to the description method. Thus, by default, an object’s debug description is the same as its description. However, you can override debugDescription if you want to decouple these.

Availability

See Also

Sends a message to the receiver with an object as the argument. (required)

Declaration

Objective-C

- (id)performSelector:(SEL)aSelectorwithObject:(id)anObject

Parameters

aSelector

A selector identifying the message to send. If aSelector is NULL, an NSInvalidArgumentException is raised.

anObject

An object that is the sole argument of the message.

Return Value

An object that is the result of the message.

Discussion

This method is the same as performSelector: except that you can supply an argument for aSelector. aSelector should identify a method that takes a single argument of type id. For methods with other argument types and return values, use NSInvocation.

Declaration

Parameters

A selector identifying the message to send. If aSelector is NULL, an NSInvalidArgumentException is raised.

anObject

An object that is the first argument of the message.

anotherObject

An object that is the second argument of the message

Return Value

An object that is the result of the message.

Discussion

This method is the same as performSelector: except that you can supply two arguments for aSelector. aSelector should identify a method that can take two arguments of type id. For methods with other argument types and return values, use NSInvocation.

Import Statement

Availability

See Also

Returns a Boolean value that indicates whether the receiver does not descend from NSObject. (required)

Declaration

Swift

funcisProxy() -> Bool

Objective-C

- (BOOL)isProxy

Return Value

NOfalse if the receiver really descends from NSObject, otherwise YEStrue.

Discussion

This method is necessary because sending isKindOfClass: or isMemberOfClass: to an NSProxy object will test the object the proxy stands in for, not the proxy itself. Use this method to test if the receiver is a proxy (or a member of some other root class).

As a convenience, retain returns self because it may be used in nested expressions.

You would implement this method only if you were defining your own reference-counting scheme. Such implementations must return self and should not invoke the inherited method by sending a retain message to super.

Import Statement

Availability

Declaration

Objective-C

- (oneway void)release

Discussion

The receiver is sent a dealloc message when its reference count reaches 0.

You would only implement this method to define your own reference-counting scheme. Such implementations should not invoke the inherited method; that is, they should not include a release message to super.

Import Statement

Availability

Declaration

Return Value

The receiver’s reference count.

Special Considerations

This method is of no value in debugging memory management issues. Because any number of framework objects may have retained an object in order to hold references to it, while at the same time autorelease pools may be holding any number of deferred releases on an object, it is very unlikely that you can get useful information from this method.

To understand the fundamental rules of memory management that you must abide by, read Memory Management Policy. To diagnose memory management problems, use a suitable tool:

The Clang Static analyzer can typically find memory management problems even before you run your program.